The present invention relates generally to gas turbines, for example, for electrical power generation, and more particularly to cooling the stage one nozzles of such turbines. The invention relates in particular to an insert design for a gas turbine nozzle cavity that provides for both convection and impingement cooling.
The traditional approach for cooling turbine blades and nozzles was to extract high pressure cooling air from a source, for example, from the intermediate and final stages of the turbine compressor. In such a system, a series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. In contrast, external piping is used to supply air to the nozzles, with air film cooling typically being used and the air exiting into the hot gas stream of the turbine. In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It was therefore necessary to establish a cooling scheme to protect the hot gas path components during operation. Steam has been demonstrated to be a preferred cooling media for cooling gas turbine nozzles (stator vanes), particularly for combined-cycle plants. See, for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. For a complete description of the steam-cooled buckets, reference is made to U.S. Pat. No. 5,536,143, the disclosure of which is incorporated herein by reference. For a complete description of the steam (or air) cooling circuit for supplying cooling medium to the first and second stage buckets through the rotor, reference is made to U.S. Pat. No. 5,593,274, the disclosure of which is incorporated herein by reference.
Because steam has a higher heat capacity than the combustion gas, however, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, in conventional steam cooled buckets it has been considered desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Nevertheless, certain areas of the components in the hot gas path cannot practically be cooled with steam in a closed circuit. For example, the relatively thin structure of the trailing edge of the nozzle vane effectively precludes steam cooling of that edge. Accordingly, air cooling is used to cool those portions of the nozzle vanes. For a complete description of the steam cooled nozzles with air cooling along the trailing edge, reference is made to U.S. Pat. No. 5,634,766, the disclosure of which is incorporated herein by reference. The flow of cooling air in a trailing edge cavity per se is the subject of a U.S. Pat. No. 5,611,662, the disclosure of which is incorporated herein by reference.
In the closed circuit system, a plurality of nozzle vane segments are provided, each of which comprises one or more nozzle vanes extending between inner and outer side walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner side walls for flowing cooling media in a closed circuit for cooling the outer and inner walls and the vanes per se. Thus, cooling media may be provided to a plenum in the outer wall of the segment for distribution to chambers therein and passage through impingement openings in a plate for impingement cooling of the outer wall surface of the segment. The spent impingement cooling media flows into leading edge and aft cavities extending radially through the vane. At least one cooling fluid return/intermediate cooling cavity extends radially and lies between the leading edge and aft cavities. A separate trailing edge cavity may also provided.
Conventionally, in each of the leading edge, intermediate and aft cavities, inserts are provided, having impingement flow holes. Thus, impingement cooling is typically provided in the leading and aft cavities of the vane, as well as in the return cavities of the first stage nozzle vane. The inserts in the leading and aft cavities comprise sleeves having a collar at their inlet ends for connection with integrally cast flanges in the outer wall and extend through the cavities spaced from the walls thereof. The inserts have impingement holes in opposition to the walls of the cavity whereby steam or air flowing into the inserts flows outwardly through the impingement holes for impingement cooling of the vane walls. Similarly, inserts in the return intermediate cavities have impingement openings for flowing impingement cooling medium against the side walls of the vane.
A problem encountered in conventional closed circuit cooled turbine nozzles, whether air or steam is used as the coolant, is that the post impingement coolant can become cross flow and reduce the effectiveness of more downstream impingement cooling. This also causes uncertainty in the calculations used to determine the cross flow effect upon heat transfer coefficient along the cavity.
Another problem encountered in conventional nozzle cavity impingement cooling systems is that due to the significant post impingement cross flow in a small cavity, a large pressure drop is needed to achieve adequate heat transfer coefficients. This large pressure drop results in a more complex design of other parts of the nozzle cooling circuit, to balance the pressure drop from other branches of the closed circuit. In most cases, excessive pressure drop from the cooling flow may not be possible due to other restrictions in the design. Reducing this pressure drop would allow for more simplified designs elsewhere in the flow circuit. It may also be required for the system to operate efficiently.
One way in which this cross flow problem has been partially addressed is to define ribs oriented generally transverse to the radial extent of the nozzle cavities so that post impingement coolant flows in a chord-wise direction to a post impingement cooling flow channel for passage to the radially inner wall of the vane segment. However, it would be desirable to address the foregoing problems associated with current nozzle insert design in a manner that would simplify the design of the vane cavity and the insert, reduce or eliminate the cross flow effect and reduce the uncertainty associated with the design.
The inventors have recognized that reducing the amount of impingement, or changing it from impingement cooling to convective cooling, will reduce or eliminate the cross flow effect and reduce the uncertainty associated with the design. More specifically, the present invention provides a novel cavity insert design wherein the amount of impingement flow is reduced so that the cooling provided along a portion of the length of the nozzle cavity is changed from impingement cooling to convective cooling. This reduces or eliminates the cross-flow effect and reduces the uncertainty associated with the design.
Accordingly, in an embodiment of the present invention, there is provided a closed circuit stator vane segment comprising radially inner and outer walls spaced from one another, a vane extending between the inner and outer walls and having leading and trailing edges and pressure and suction sides, the vane including discrete cavities between the leading and trailing edges and extending lengthwise of the vane, and an insert sleeve in at least one of those cavities, the insert sleeve having impingement holes for directing the cooling media against interior wall surfaces of the cavity. The impingement holes are defined in first and second walls of the insert sleeve facing respectively the pressure and suction sides of the vane. However, the impingement holes of at least one of those first and second walls are defined along substantially only a first, upstream portion thereof whereby the cooling flow is predominantly impingement cooling along the first, upstream portion and the cooling flow is predominantly convective cooling along a second, downstream portion thereof.
In a currently preferred embodiment, the impingement holes of both the first and second walls of the insert sleeve extend along substantially only respective first, upstream portions thereof so that there is a transition to convective cooling along both those walls. Even more preferably, the impingement holes in the second wall, facing the suction side of the vane extend along a lesser extent of that wall than the impingement holes in the first wall.